This invention relates to a method and an apparatus for detecting a sampling-period sync signal from an output signal of a digital-to-analog converter. This invention also relates to a recording apparatus and a reproducing apparatus.
Some audio recording and reproducing devices have the function of reproducing a digital signal from a recording medium, converting the digital signal into a corresponding analog signal, and outputting the analog signal. General DATs (digital audio tape recorders) have the function of receiving an analog signal, converting the analog signal into a corresponding digital signal, and recording the digital signal into a recording medium.
In some cases, a digital signal is reproduced from a recording medium and is converted into a corresponding analog signal, and then the analog signal is converted back into a corresponding digital signal and the digital signal is recorded again into a recording medium by a DAT. In these cases, the quality of the recorded digital signal is lower than the quality of the original digital signal.
It was experimentally found that a main cause of the deterioration of the recorded digital signal was the presence of an anti-aliasing filter in these recording and reproducing devices. The anti-aliasing filter includes a low pass filter which prevents high-frequency signal components from causing aliasing during a sampling process in analog-to-digital conversion. The anti-aliasing filter is composed of either an analog filter or a digital filter. Generally, an analog-type anti-aliasing filter has poor group delay frequency characteristics which cause a degradation of a filtered signal. An FIR digital-type anti-aliasing filter has good group delay frequency characteristics but causes unwanted signal components such as pre-echoes and after-echoes.
In cases where an anti-aliasing filter is removed and the output signal from a digital-to-analog converter of a reproducing side is directly fed to an analog-to-digital converter of a recording side, a good signal quality can be ensured only when the operation of the digital-to-analog converter and the operation of the analog-to-digital converter are accurately synchronized with each other. To synchronize the analog-to-digital converter of the recording side with the digital-to-analog converter of the reproducing side, it is necessary to detect a sampling-period sync signal from an output signal of the digital-to-analog converter of the reproducing side.